Non ablative, state changeable, optical data storage systems record information in a state changeable material that is switchable between at least two detectable states by the application of energy thereto, for example, the application of projected beam energy such as optical energy, particle beam energy, or the like.
The state changeable optical data storage material is present in an optical data storage device having a structure such that the data storage material is supported by a substrate and encapsulated in encapsulants. In the case of an optical data storage device, the encapsulants may include anti-ablation materials and layers, thermal insulating materials and layers, anti-reflection layers between the projected beam source and the data storage medium, reflective layers between the optical data storage medium and the substrate, and the like. Various layers may perform more than one of these functions. For example, the anti-reflection layers may also be thermal insulating layers. The thicknesses of the layers, including the layer of state changeable data storage material, are optimized whereby to minimize the energy necessary for state change while retaining the high contrast ratio, high signal to noise ratio, and high stability of the state changeable data storage material.
The state changeable material is a material capable of being switched from one detectable state to another detectable state by the application of projected beam energy thereto. State changeable materials are such that the detectable states may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties, and combinations of these properties. The state of the state changeable material is detectable by the electrical conductivity, electrical resistivity, optical transmissivity, optical absorbtion, optical reflectivity and any combination thereof.
In the case of optical data storage materials, the optical data storage material is typically deposited as a disordered material and formed or initialized to a system having (1) relatively reproducible, erased or "0", relatively ordered or even crystalline properties, (2) relatively reproducible written, binary "1", relatively disordered or even amorphous detectable properties, preferably with (3) a relatively high degree of history invariant discrimination therebetween for a high number of write-erase cycles, i.e. for a relatively high number of vitrify-crystallize cycles, (4) a relatively high crystallization velocity; and (5) a relatively high degree of thermal stability in both states.
Deposition may be by evaporative deposition, chemical vapor deposition, or plasma deposition. As used herein plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition. In many cases, the resulting as deposited disordered material must be initialized as described, for example, in the commonly assigned copending application of Rosa Young and Napoleon Formigoni for Method Of Forming An Optical Data Storage Device Ser. No. 769,227, filed Aug. 26, 1985, a continuation-in-part of commonly assigned, copending U.S. application Ser. No. 667,294, filed Nov. 1, 1984, and now abandoned. That is, the memory must be conditioned, formed, initialized, or otherwise prepared to receive data if the data is going to be recorded in a disordered (binary "1") state. Initialization, i.e. formation, requires the conversion of the phase changeable data storage material from the as deposited disordered or ordered state to a stable system switchable between a vitrified, disordered, written state corresponding to binary 1 and an ordered "erased", crystallized state corresponding to binary "0" with history invariant cycling properties.
Tellurium based materials have been utilized as phase changeable memory materials. This effect is described, for example, in J. Feinleib, J. deNeufville, S. C. Moss, and S. R. Ovshinsky, "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors", Appl. Phys. Lett., Vol. 18 (6), pages 254-257 (Mar. 15, 1971), in J. Feinleib, S. Iwasa, S. C. Moss, J. P. deNeufville, and S. R. Ovshinsky, "Reversible Optical Effects In Amorphous Semiconductors", Journal of Non-Crystalline Solids, Vol. 8-10, pages 909-916 (1972), and in U.S. Pat. No. 3,530,441 to S. R. Ovshinsky for Method and Apparatus For Storing And Retrieving Of Information. A recent description of tellurium-germanium-tin systems, without oxygen, is in M. Chen, K. A. Rubin, V. Marrello, U. G. Gerber, and V. B. Jipson, "Reversibility And Stability of Tellurium Alloys for Optical Data Storage," Appl. Phys. Lett., Vol. 46 (8), pages 734-736 (Apr. 15, 1985).
Tellurium based state changeable materials, in general, are multiphase systems (1) where the ordering phenomena includes nucleation and growth processes (both homogeneous and heterogeneous) to convert a system of disordered materials to a system of ordered and disordered materials, and (2) where the vitrification phenomena includes melting and rapid solidification of the phase changeable material to transform a system of disordered and ordered components to a system of disordered components. The above phase changes and separations occur over relatively small distances with intimate interlocking of the phases and gross structural discrimination.
The major limitation of using state change materials for optical data storage is the trade off between thermal stability and the crystallization rate. Another limitation is the cycle history dependency of the contrast ratio as interfacial interactions occur between phase changeable materials. A more subtle limitation is the observed increase in the "erased" signal to noise ratio with respect to time in the written state prior to erasure. Reflectivity is a function of crystallite orientation. The "erased" signal to noise ratio increases as certain preferred nucleations initiate and/or nucleation site rearrangements occur during room temperature storage.